Image sensor with dual element color filter array and three channel color output

ABSTRACT

A color image sensor includes a pixel array including a CFA overlaying an array of photo-sensors for acquiring a color image. The CFA includes first color filter elements of a first color overlaying a first group of the photo-sensors, second color filter elements of a second color overlaying a second group of the photo-sensors, and a plurality of filter stacks overlaying a third group of the photo-sensors. The first group generates first color signals of a first color channel and the second group generates second color signals of a second color channel. Each of the filter stacks includes a first stacked filter of the first color and a second stacked filter of the second color. A sensitivity of the filter stacks equals a product of sensitivities of the first and the second stacked filters and the filter stacks generate a third color channel.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a Divisional of U.S. patent application Ser.No. 12/842,808, filed on Jul. 23, 2010.

TECHNICAL FIELD

This disclosure relates generally to image sensors, and in particularbut not exclusively, relates to color image sensors.

BACKGROUND INFORMATION

Color image sensors are used in various applications from simple andinexpensive mobile-phone cameras and digital cameras to sophisticatedand expensive medical and scientific equipment. Conventional color imagesensors include a pixel array having a color filter array (“CFA”)overlaying an array of photosensitive elements, which convert theintensity of the color filtered light into electrical signals.

The CFA typically includes three color filter elements for generating athree channel color output. Often three primary color filters are usedto generate three primary color signals: red (R), green (G), and blue(B) signals. Thus, each pixel in the pixel array is sensitive to one ofthe three primary colors and outputs a corresponding color signal. Forexample, to produce a R signal, the pixel is covered by a red colorfilter element, to produce a G signal, the pixel is covered by a greencolor filter element, and to produce a B signal, the pixel is covered bya blue color filter element. Accordingly, at least three different colorfilter elements are used. The three different color filter elements usethree separate fabrication procedures to fabricate the complete threecolor element CFA.

FIG. 1 illustrates an example Bayer pattern CFA 10 formed using Rpixels, G pixels, and B pixels. CFA 10 generates a three channel coloroutput of R, G, and B signals. CFA 10 may have hundreds or eventhousands of rows and columns. The Bayer pattern CFA uses a repeatingpattern of macropixel groups 20. Each macropixel group 20 includes fourmicropixels 30: a R pixel, a B pixel, and two G pixels. A singlemacropixel group 20 produces a R signal, a G signal (the two G signalsare combined), and a B signal. Each macropixel group 20 may beconsidered the smallest element of a color image produced by the colorimage sensor, where each macropixel group 20 corresponds to a singleimage pixel in the output image data. In other words, the resolution ofthe output image directly corresponds to the linear density ofmacropixel groups 20. The R, G, and B signals output by the individualmicropixels 30 of a single macropixel group 20 are combined to form anindividual color image pixel in the output color image data.

The extraction of R, G, and B signals from each macropixel group 20 isstraightforward. The R pixel produces the R signal, the G pixels producethe G signal, and the B pixel produces the B signal. Of course, the Gsignal is larger than either R or B signal for the same light intensity,which mimics the human eye's higher sensitivity to green light comparedto red and blue light. Thus, conventional color image sensors have aone-to-one correspondence between the number of distinct colors in thefilter elements of the CFA and the number of output color channels.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the invention aredescribed with reference to the following figures, wherein likereference numerals refer to like parts throughout the various viewsunless otherwise specified.

FIG. 1 (PRIOR ART) illustrates a conventional Bayer pattern color filterarray (“CFA”).

FIG. 2 is a functional block diagram illustrating a color imaging systemincluding a pixel array having a CFA with two color filter elements, butwhich imaging system is capable of generating a three channel coloroutput, in accordance with an embodiment of the invention.

FIG. 3 is a schematic illustrating normalized sensitivity functions forred, green, blue, cyan, magenta, and yellow signals, in accordance withan embodiment of the invention.

FIG. 4 is a schematic illustrating how the green sensitivity function isa product of the cyan and yellow sensitivity functions, in accordancewith an embodiment of the invention.

FIG. 5 is a graph illustrating sensitivity functions of cyan and yellowcolor filter elements, in accordance with an embodiment of theinvention.

FIGS. 6A-6C illustrate the fabrication of a macropixel group using twotypes of color filter elements to generate image signals, which can becombined to generate a three channel color output, in accordance with anembodiment of the invention.

FIGS. 7A-7C illustrate the fabrication of a macropixel group using twotypes of color filter elements capable of directly outputting threecolor channels, in accordance with an embodiment of the invention.

FIG. 8 is a schematic illustrating how the blue sensitivity function isa product of the cyan and magenta sensitivity functions, in accordancewith an embodiment of the invention.

FIG. 9 is a schematic illustrating how the red sensitivity function is aproduct of the magenta and yellow sensitivity functions, in accordancewith an embodiment of the invention.

FIG. 10 is a circuit diagram illustrating pixel circuitry of two 4Tpixels within an imaging system, in accordance with an embodiment of theinvention.

DETAILED DESCRIPTION

Embodiments of an apparatus, and methods of operation and fabrication ofthe same, for an image sensor with dual element color filter array(“CFA”) and three channel color output are described herein. In thefollowing description numerous specific details are set forth to providea thorough understanding of the embodiments. One skilled in the relevantart will recognize, however, that the techniques described herein can bepracticed without one or more of the specific details, or with othermethods, components, materials, etc. In other instances, well-knownstructures, materials, or operations are not shown or described indetail to avoid obscuring certain aspects.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearances of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

FIG. 2 is a block diagram illustrating a color imaging system 100 (alsoreferred to as color image sensor 100), in accordance with an embodimentof the invention. The illustrated embodiment of color imaging system 100includes a pixel array 105, readout circuitry 110, control circuitry115, and an output port 125. The illustrated embodiment further includesa color signal combining circuit (“CSCC”) 120, though some embodimentsmay operate without CSCC 120 or with CSCC 120 disabled or by-passed. Inone embodiment, the illustrated components of color imaging system 100are all integrated on a single semiconductor die that has a CFA withonly two types of color filter elements, but outputs from the die threechannel color image data.

Pixel array 105 is a two-dimensional (“2D”) array of pixels (e.g.,pixels P1, P2 . . . , Pn). In one embodiment, each pixel is acomplementary metal-oxide-semiconductor (“CMOS”) imaging pixel. Asillustrated, each pixel is arranged into a row (e.g., rows R1 to Ry) anda column (e.g., column C1 to Cx) to acquire image data of a person,place, or object, which can then be used to render a 2D image of theperson, place, or object. In one embodiment, pixel array 105 is abackside illuminated (“BSI”) image array. In one embodiment, pixel array105 is a frontside illuminated (“FSI”) image array. Pixel array 105includes a pattern of individual color filter elements or color filterarray (“CFA”) disposed over the light incident side of pixel array 105.The color filter element pattern repeats within each macropixel group135 across pixel array 105. Each macropixel group 135 is made up ofmultiple individual pixels P1-Pn (also referred to as micropixels toemphasis their relationship as members of a given macropixel group). Inone embodiment, each macropixel group 135 corresponds to an individualimage pixel within the output color image 137 and includes four pixels(e.g., P1-P4, P5-P8, etc.)

After each pixel has acquired its image data or image charge, the imagedata is readout by readout circuitry 110. Readout circuitry 110 mayinclude amplification circuitry, analog-to-digital (“ADC”) conversioncircuitry, or otherwise. In one embodiment, readout circuitry 110 mayreadout a row of image data at a time along readout column lines(illustrated) or may readout the image data using a variety of othertechniques (not illustrated), such as a serial readout or a fullparallel readout of all pixels simultaneously.

Embodiments of the present invention use a color combiner unit tocombine two color channels to generate three color channels for a colorimage. In a first embodiment, the color combiner unit includes circuitry(e.g., digital logic, analog summation circuitry, analog multiplicationcircuitry, other analog/digital circuitry, microprocessor andsoftware/firmware logic, etc.) for combining two color signals output bypixel array 105 to generate three color channel image data. In a secondembodiment, the color combiner unit is implemented using filter stacksof two separate color filters to optically combine the color channelsassociated with each of the color filter elements to generate thirdcolor channel. In this second embodiment, pixel array 105 outputs threecolor signals.

In one embodiment, pixel array 105 outputs image signals on readoutlines 140 to readout circuitry 110 that includes two secondary colorchannels, such as a cyan (C) channel and a yellow (Y) channel(illustrated), a C channel and a magenta (M) channel, an M channel and aY channel, or otherwise. This two channel color data is then input intoCSCC 120, which is a color combiner unit combining the two channel imagedata to generate three channel image data for color image 137. Forexample, the three channel image data may include C, Y, and green (G)color channels (illustrated); C, M, and blue (B) color channels; M, Y,and red (R) color channels; or otherwise. In one embodiment, CSCC 120includes further circuitry to transform two secondary color channelsinto three primary color channels R/G/B, which is a common color imageformat.

In one embodiment, pixel array 105 outputs image signals on readoutlines 140 to readout circuitry 110 that includes three color channels.In this embodiment, CSCC 120 may not be included or otherwisedisabled/by-passed. Alternatively, CSCC 120 may be enabled to transformthree color channels from pixel array 105 to the three channel primarycolor combination, or even transform from one combination including twosecondary colors to another combination including different secondarycolors. Examples of three color channel combinations output directlyfrom pixel array 105 include C/Y/G, C/M/B, M/Y/R, or otherwise.

Although CSCC 120 is illustrated as being separate from readoutcircuitry 110, in some embodiments, the functionality and/or logic ofCSCC 120 may be entirely or partially incorporated into readoutcircuitry 110. In one embodiment, CSCC 120 may even be coupledexternally to the die upon which pixel array 105, readout circuitry 110,and control circuitry 115 are integrated. In one embodiment, CSCC 120 isimplemented using hardware. In one embodiment, CSCC 120 may even beimplemented partially or entirely in software.

Control circuitry 115 is coupled to pixel array 105 to controloperational characteristics of pixel array 105. For example, controlcircuitry 115 may generate a shutter signal for controlling imageacquisition. In one embodiment, the shutter signal is a global shuttersignal for simultaneously enabling all pixels within pixel array 105 tosimultaneously capture their respective image data during a singleacquisition window. In an alternative embodiment, the shutter signal isa rolling shutter signal whereby each row, column, or group of pixels issequentially enabled during consecutive acquisition windows.

Embodiments of the present invention may implement macropixels 135 usingvarious combinations of just two color filter elements, as opposed tothe three color filter elements used in the conventional Bayer pattern.The R, G, B, colors used in the Bayer pattern are referred to as primarycolors or primary color channels while the combination of R, G, & B isreferred to as the primary color set. The Bayer pattern may also usealternative secondary colors such as cyan (C), magenta (M), and yellow(Y). The primary colors are related to the secondary colors according toequations (Eq. 1) to (Eq. 6) below.C=B+G  (Eq. 1)M=B+R  (Eq. 2)Y=R+G  (Eq. 3)R=(Y+M−C)/2  (Eq. 4)G=(Y−M+C)/2  (Eq. 5)B=(C+M−Y)/2  (Eq. 6)

Based on the above equations, to get R, G, and B signals, conventionalCFAs used three separate R, G, and B, color filter elements or threeseparate C, M, and Y color filter elements. In either conventional case,three separate color filters elements are used, which requires threedistinct processing procedures during fabrication to lay down the threedifferent colored filter materials.

Eqs. (1) to (3) are schematically represented in FIG. 3, where thevertical axis of each chart is a normalized sensitivity and thehorizontal axis is a wavelength. FIG. 4 illustrates that themultiplication of the C sensitivity function and the Y sensitivityfunction results in the G sensitivity function (e.g., G=C*Y). Therefore,a G color signal can be produced from the C signal and the Y signal(i.e., G=C*Y) output by the photo-sensors overlaid by a C color filterelement and Y color filter element, respectively. Furthermore, referringto Eq. (1), a B signal can be produced from the C color signal and Gcolor signal (i.e., B=C−G) and the R color signal can be produced fromthe Y color signal and the G color signal (i.e., R=Y−G). FIG. 5illustrates two example sensitivity functions for a C color filterelement and a Y color filter element, respectively. The product of the Cand Y sensitivity functions, which is the G sensitivity function, isalso illustrated.

FIGS. 6A-6C illustrate the fabrication of a macropixel group using twotypes of color filter elements to generate two channel image signals,which can be combined to generate a third channel for a three channelcolor output, in accordance with an embodiment of the invention.Macropixel group 600 is one possible implementation of macropixel groups135 illustrated in FIG. 2. Macropixel group 600 includes four pixels605, 610, 615, and 620. Pixels 605 and 620 include separatephoto-sensors 630 (only one is illustrated in the cross-sectional viewof FIG. 6C) covered by a color filter element 631 having a first color(e.g., a cyan color filter element is illustrated as an example). Pixels610 and 615 include photo-sensors 635 covered by a color filter element636 having a second color different from the first color (e.g., a yellowcolor filter element). FIG. 6A illustrates a first fabrication stepwhere the first color filter layer is disposed over the pixel array andpatterned to cover pixels 605 and 620 while removed from pixels 610 and615. FIG. 6B illustrates a second fabrication step where the secondcolor filter layer is formed over the remaining exposed pixels 610 and615. FIG. 6C illustrates a cross-sectional view along line A-A′ in FIG.6B.

In the illustrated embodiment of FIGS. 6A-C, pixels 605 and 620contribute to the C color signal. However, the C color signal may befrom pixel 605, pixel 620, or the combination of both. Pixels 610 and615 contribute to the Y color signal. However, the Y color signal may befrom pixel 610, pixel 615, or the combination of both. The G colorsignal is generated based upon the product of the C color signal and theY color signal. During operation, pixel array 105, implemented withmacropixel groups 600, outputs image data having two types of colorsignals (i.e., two color channel image data). This two color channelimage data is readout by readout circuitry 110 and provided to CSCC 120.In one embodiment, CSCC 120 includes operation for combining the twocolor image signals to generate an additional third color image signalsuch that a three channel image signal can be output from output port125. In the example of C and Y color filter elements, CSCC 120 mayinclude a multiplier to multiply the C and Y color signals to generate aG color signal. However, as discussed below, other combinations of colorfilter elements may require addition or subtraction functions togenerate the third color channel signal based upon the two colorchannels output from pixel array 105.

In one embodiment, CSCC 120 may further include a color transformer sothat secondary color signals (e.g., C, M, Y) can be transformed intoprimary color signals (e.g., R, G, B), or even to facilitatetransformation between two different sets of secondary color signals. Inthis manner, color imaging system 100 may be capable of selecting andoutputting a variety of different three channel color sets (e.g., RGB,CYG, CMB, MYR, or otherwise). The transformer may include additionand/or subtraction functions, as discussed below. For example, if themacropixel groups 600 include C and Y color filter elements and the Gcolor signal is generated by CSCC 120 via multiplication of the C and Ycolor signals, then the transformer function included within CSCC 120may include a difference function to generate a B color signal via C−G(Eq. 1) or generate a R color signal via Y−G (Eq. 2).

FIGS. 7A-7C illustrate the fabrication of a macropixel group 700 usingtwo types of color filter elements capable of directly outputting threecolor channels, in accordance with an embodiment of the invention.Macropixel group 700 is one possible implementation of macropixel groups135 illustrated in FIG. 2. Macropixel group 700 includes four pixels705, 710, 715, and 720. Pixels 710 and 715 include photo-sensors 730(only one is illustrated in the cross-sectional view of FIG. 7C) eachcovered by two color filter elements 731 and 732 each having a differentcolor (e.g., color filter element 731 is a cyan color and color filterelement 732 is yellow, other color combination are possible). Pixels 705and 720 each include a photo-sensor covered by a single type of colorfilter element. In the illustrated embodiment, pixel 705 is covered by aC color filter element while pixel 720 is covered by a Y color filterelement. In one embodiment, a clear planarization layer may be disposedover pixels 705 and 720 such that the top surface of pixel array 105 islevel for application of microlenses.

FIG. 7A illustrates a first fabrication step where the first colorfilter layer (e.g., cyan color filter material) is disposed over thepixel array and patterned to cover three pixels of each four pixelmacropixel group 700 (e.g., pixels 705, 710, and 715) while removed fromone pixel in macropixel group 700 (e.g., pixel 720). FIG. 6B illustratesa second fabrication step where the second color filter layer (e.g.,yellow color filter material) is formed over three of the pixels withineach macropixel group 700 (e.g., pixels 710, 715, and 720). This resultsin two of the pixels including a color filter stack of both color filterelements. FIG. 6C illustrates a cross-sectional view along line B-B′ inFIG. 7B. In the illustrated embodiment, the Y color filter element isleveled with the adjacent filter stack using a clear planarizationelement (PL).

In the embodiment illustrated in FIGS. 7A-C, pixels 710 and 715 arecovered by two different types of color filter elements. The sensitivityof pixels 710 and 715 is the product of these two different types ofcolor filter elements. Stacking two different color filter elementscauses an optical multiplication effect, similar to multiplying twodifferent color signals. In the illustrated embodiment, pixels 710 and715 contribute to generate a G color signal, since the G sensitivityfunction is the product of the C sensitivity function and the Ysensitivity function.

Thus, pixel array 105 implemented with embodiments of macropixel group700 is capable of outputting image data having three different types ofcolor signals (i.e., three color channel image data) with only twodistinct types of color filter elements. In one embodiment, CSCC 120 mayinclude a transformer to transform image data readout of pixel array 105to a different three color channel set. For example, color image system100 may output a three color channel signal including the primary colorRGB. In this case, the transformer may generate the B color signal basedon the difference of the C and G color signals and generate the R colorsignal based on the difference of the Y and G color signals.

It should be appreciated that the order of the color filter elements canbe reversed (e.g., cyan on top and yellow on bottom). Furthermore, insome embodiments, only a single pixel within macropixel group 700includes a dual color filter stack, while two of the other pixelscovered by a single color filter may have their output color signalscombined to increase the sensitivity towards that particular color.

Returning to FIG. 5, embodiments of the present invention appear toreduce color crosstalk between the G color signal and the individual Cor Y color signals, since the G color signal is the product of theindividual C and Y color signals. Furthermore, the G sensitivityfunction may be optimized for a given application by extending orvarying the bandwidth of the G sensitivity function by selectingappropriate C and Y sensitivity functions.

In one embodiment, a pair of pixels (e.g., 605 and 610) may be logicallygrouped to form a macropixel group (e.g., the smallest logical groupingof pixels that contribute to a single pixel within color image 137). Asseen in FIGS. 6A-C, only two pixels are needed to generate three colorchannels. As such, the resolution of pixel array 105 can be increased byreassigning a given macropixel group 135 to include just two pixels.

Pixel array 105 may be implemented with alternative color filter elementcombinations. For example, a set of C and M secondary color filterelements may be used. FIG. 8 is a schematic illustrating how the Bsensitivity function is the product of the C and M sensitivityfunctions. Therefore, a B color signal can be produced from a C colorsignal and an M color signal (i.e., B=C*M). Referring to Eq. (1), a Gcolor signal can be produced from a C color signal and a B color signal(i.e., G=C−B), and referring to Eq. (2) a R color signal can be producedfrom a M color signal and a B color signal (i.e., R=M−B).

Similarly, a set of M and Y secondary color filter elements may be used.FIG. 9 is a schematic illustrating how the R sensitivity function is aproduct of the M and Y sensitivity functions. Therefore, an R colorsignal can be produced from an M color signal and a Y color signal(i.e., R=M*Y). Referring to Eq. (2), a B color signal can be producedfrom an M color signal and a R color signal (i.e., B=M−R), and referringto Eq. (3) a G color signal can be produced from a Y color signal and anR color signal (i.e., G=Y−R).

FIG. 10 is a circuit diagram illustrating pixel circuitry 1000 of twofour-transistor (“4T”) pixels within a pixel array, in accordance withan embodiment of the invention. Pixel circuitry 1000 is one possiblepixel circuitry architecture for implementing each pixel within pixelarray 105 of FIG. 2. However, it should be appreciated that embodimentsof the present invention are not limited to 4T pixel architectures;rather, one of ordinary skill in the art having the benefit of theinstant disclosure will understand that the present teachings are alsoapplicable to 3T designs, 5T designs, and various other pixelarchitectures. In FIG. 10, pixels Pa and Pb are arranged in two rows andone column. The illustrated embodiment of each pixel circuitry 1000includes a photo-sensor (e.g., photodiode PD), a transfer transistor T1,a reset transistor T2, a source-follower (“SF”) transistor T3 and aselect transistor T4. During operation, transfer transistor T1 receivesa transfer signal TX, which transfers the charge accumulated inphotodiode PD to a floating diffusion node FD. In one embodiment,floating diffusion node FD may be coupled to a storage capacitor fortemporarily storing image charges. Reset transistor T2 is coupledbetween a power rail VDD and the floating diffusion node FD to reset(e.g., discharge or charge the FD to a preset voltage) under control ofa reset signal RST. The floating diffusion node FD is coupled to controlthe gate of SF transistor T3. SF transistor T3 is coupled between thepower rail VDD and select transistor T4. SF transistor T3 operates as asource-follower providing a high impedance output from the pixel.Finally, select transistor T4 selectively couples the output of pixelcircuitry 1000 to the readout column line under control of a selectsignal SEL. In one embodiment, the TX signal, the RST signal, and theSEL signal are generated by control circuitry 115.

The above description of illustrated embodiments of the invention,including what is described in the Abstract, is not intended to beexhaustive or to limit the invention to the precise forms disclosed.While specific embodiments of, and examples for, the invention aredescribed herein for illustrative purposes, various modifications arepossible within the scope of the invention, as those skilled in therelevant art will recognize. For example, although examples of CMOScolor image sensors are illustrated, the embodiments may include othertypes of color image sensor such as charge coupled devices (CCDs) andothers that use three distinct color filters to generate a three colorchannel signal.

These modifications can be made to the invention in light of the abovedetailed description. The terms used in the following claims should notbe construed to limit the invention to the specific embodimentsdisclosed in the specification. Rather, the scope of the invention is tobe determined entirely by the following claims, which are to beconstrued in accordance with established doctrines of claiminterpretation.

What is claimed is:
 1. A color image sensor comprising: a pixel arrayincluding a color filter array (“CFA”) overlaying an array ofphoto-sensors for acquiring a color image, wherein the CFA includes:first color filter elements of a first color overlaying a first group ofthe photo-sensors, wherein the first group generates first color signalsof a first color channel; second color filter elements of a second coloroverlaying a second group of the photo-sensors, wherein the second groupgenerates second color signals of a second color channel; and aplurality of filter stacks overlaying a third group of thephoto-sensors, wherein each of the filter stacks includes a firststacked filter of the first color and a second stacked filter of thesecond color, wherein a sensitivity of the filter stacks equals aproduct of sensitivities of the first and the second stacked filters ,and wherein the filter stacks generate a third color channel, whereinthe color image sensor is integrated on a semiconductor die, wherein thecolor image sensor further includes an output port coupled to the pixelarray to output the color image having three color channels off of thesemiconductor die.
 2. The color image sensor of claim 1, furthercomprising: readout circuitry coupled between the pixel array and theoutput port to readout the color image having the three color channelsfrom the pixel array.
 3. The color image sensor of claim 1, wherein: thefirst color filter elements comprise cyan color filters, the secondcolor filter elements comprise yellow color filters, and the first andsecond filter elements included within the filter stacks to form acombination green color filter.
 4. The color image sensor of claim 1,wherein: the first color filter elements comprise magenta color filters,the second color filter elements comprise yellow color filters, and thefirst and second filter elements included within the filter stacks toform a combination red color filter.
 5. The color image sensor of claim1, wherein: the first color filter elements comprise cyan color filters,the second color filter elements comprise magenta color filters, and thefirst and second filter elements included within the filter stacks toform a combination blue color filter.
 6. The color image sensor of claim2, further comprising: a color signal combiner circuit coupled betweenthe readout circuitry and the output port to transform the first,second, and third color channels of the color image readout from thepixel array from a first set of three colors to a second set of threecolors.
 7. The color image sensor of claim 5, wherein the first set ofthree colors comprise a secondary color set and the second set of threecolors comprises a primary color set including Red, Green, and Bluecolor channels for output from the color image sensor.